github: Stop explicitly installing gnu-tar on macOS

These days, it seems to be installed out of the box already on the
runner images, and installing it triggers very visible warnings
about it already being installed.

.github/workflows/build.yml

@@ -200,7 +200,7 @@ jobs:
           SOURCE_DATE_EPOCH: ${{needs.prepare.outputs.COMMIT_DATE_UNIX}}
           BUILD_DATE: ${{needs.prepare.outputs.BUILD_DATE}}
         run: |
-          brew install ninja gnu-tar
+          brew install ninja
           # Disable zstd and python. Both are available on the runners, but
           # installed with homebrew, and only available in the native (x86_64)
           # form. Therefore, autodetection will pick them up, but linking